This invention relates generally to a method and apparatus for interpreting data in real time, and more particularly to a method and apparatus that determine the fatigue life of a structure in real time from data relating to the stress on the structure.
Various rotating and non-rotating aircraft structures, including those that are part of an aircraft engine (e.g., a compressor or fan rotor disk), have varying lengths of service life. The service life of any structure is generally determined from the nature of the strain or physical deformation within the structure that results from operational use. In turn, the strain is determined by the pattern (i.e., magnitude, frequency) of stress forces applied to the structure over time. Further, the stress forces are determined by the operating conditions encountered by the structure. Therefore, a structure used in certain operating conditions typically has a different service life from that of the identical structure used in different operating conditions.
For any structure, the magnitude of the strain tends to be cyclic over time. Thus, the service life of the structure is generally determined from the number of strain cycles encountered by the structure while in operation. Strain cycles are generally defined by the magnitude of strain transitioning between positive and negative peak values. Over time, these cycles can cause the material comprising the structure to become fatigued, thereby ultimately causing the structure to crack and fail in operation. Thus, it is important to accurately ascertain the accrued and/or remaining service life of a structure to avoid such catastrophic effects.
The magnitude of cyclic strain within certain structures is often times alternating and/or repetitive (i.e., non-random). As such, the service life of those structures is somewhat predictable. For aircraft structures, however, the cyclic strain is most often random, due to the operating conditions of an aircraft. Strain cycles for aircraft engines in normal operation are typically determined by the number of engine speed transients and the accompanying varying temperatures and pressures. Also, more frequent and wider ranging strain cycles are prevalent in military engines than in commercial engines. This is due to the relatively many more transient operating conditions encountered by military engines during normal operation. Thus, it is generally more difficult to determine the fatigue life of a structure that is part of a military aircraft than a commercial aircraft.
Due to its inherent variation during aircraft operation, the cyclic strain within an aircraft structure and the resulting structural fatigue life cannot be accurately predicted or statistically expressed. Therefore, some means or method for determining the amount of accrued strain within an aircraft structure is needed. Also, some means or method for determining the resulting fatigue life of the structure is desired.
Early on in the prior art it was known to determine the fatigue life of an aircraft structure by having a pilot or crewmember manually record when the aircraft achieved certain operating states, such as engine start up and shut down, and aircraft takeoff and landing. The fatigue life of the structure is estimated from these manually recorded data points, often times by comparing the attained aircraft operating states to an empirically determined database.
However, this non-automatic method merely provides a rough and inaccurate approximation of the remaining service life of the aircraft structure. This method is inaccurate because it does not base its determination on operating conditions that are closely related to the service life. This most often results in the structure being replaced much sooner than it needs to be, in order to err on the side of caution. This results in needless costs expended both in parts and labor. Thus, a more accurate method and apparatus of determining the fatigue life of a structure are needed.
U.S. Pat. No. 3,979,579 discloses a processor-based system that automatically records aircraft fatigue cycles by sensing the attainment of various operating points during a typical aircraft flight. These operating points include engine startup, engine shutdown, landing gear status, engine reversal, and throttle setting. The signal processor derives full and fractional fatigue cycles from these operating points. The aircraft engine manufacturers usually define the cycles.
However, an inherent problem with the system of the ""579 patent is in its use of relatively broad, normal aircraft operating conditions in making the fatigue cycle determinations. Specifically, these operating conditions are not directly related to the actual fatigue-causing strain within the structure. Thus, the determined fatigue life of the structure is also not correlated to the strain. As a result, the system of the ""579 patent is problematic in that it likely results in an aircraft structure being replaced sooner than it has to be, in order to err on the side of caution. While the system of the ""579 patent represents an improvement over the aforementioned manual method of fatigue life determination, it is desired to have an automated system that determines the fatigue life of a structure based on a more accurate assessment of the strain that results within the structure from the stress forces imposed on the structure.
U.S. Pat. No. 4,336,595 discloses a computer system that determines the fatigue life of a structure by interpreting the time history of the strain within the structure. A sensed signal from a strain gage is input to a signal processor that determines the strain cycles encountered by the structure over time. The signal processor utilizes a modified version of the well-known xe2x80x9crainflowxe2x80x9d cycle pairing method to determine the strain cycles. In general, the rainflow method interprets the inherently relatively complex time history of the random time variations of the magnitude of the strain encountered by any type of structure. The method essentially decomposes the strain time history and counts the strain cycles utilizing several rules that define full and half cycles.
U.S. Pat. No. 5,847,668 discloses a computer system similar to that disclosed in the ""595 patent in that it senses strain data using a strain gage. The system also interprets the acquired strain data to determine the strain cycles using the rainflow method, and calculates the fatigue life of the structure.
A common feature of both the ""595 and ""668 patents is that fatigue life is based primarily on sensed data from a strain gage. Neither patent teaches the use of a temperature of the structure when determining its fatigue life. It is desired to use the temperature of the structure in determining its fatigue life, since, in general, the higher the temperature the shorter the operating life.
Also, the prior art does not teach the use of a calculated value of the stress forces imposed on a structure when determining fatigue life. A problem with strict use of a sensed strain signal in determining fatigue life occurs with rotating structures, including those commonly found in aircraft engines (e.g., a fan disk). The rotating nature of these structures generally precludes use of strain gages.
Further, the rainflow method is typically applied to the accumulated data after the conclusion of the operation of the structure (i.e., after the aircraft flight is complete). Yet, the ""595 patent purports to analyze the data in real time as it occurs using a modified version of the rainflow method. Nevertheless, the method disclosed in the ""595 patent is based on the relatively complex data interpretation rules associated with the well-known rainflow method.
An object of the present invention is to improve upon the accuracy of prior art fatigue life calculation systems by utilizing structural operating parameters that closely relate to the stress forces on a structure.
Another object of the present invention is to accurately ascertain, in real time, the fatigue life of a structure by pairing together, in real time, high and low peak data points of stress imposed on a structure.
Yet another object of the present invention is to avoid needless expense in prematurely replacing a structure well prior to the expiration of its useful life.
Still another object of the present invention is to use one or more sensed or calculated temperatures of a structure to more accurately determine the fatigue life of the structure.
Another object of the present invention is to utilize real time calculated values of the stress forces imposed on a rotating structure in determining, in real time, the fatigue life of the structure.
Yet another object of the present invention is to provide a relatively simpler method, as compared to the prior art rainflow method, of identifying, in real time, the occurrence of cycle pairs of high and low peak stress data points.
According to the present invention, a method and apparatus for determining the fatigue life of a structure calculate, in real time, the values for the magnitudes of the stress forces imposed at a particular location on the structure. The stress values are calculated from one or more associated sensed structural parameters. The temperature values of the structure at the particular location may also be calculated or measured.
The calculated stress data points are continuously examined, in real time, to determine if the direction of their magnitude is increasing (i.e., continually greater magnitude data points are being achieved) or decreasing (i.e., continually lesser magnitude data points are being achieved). If a change in direction is indicated, for example, from increasing to decreasing (i.e., the most recent data point is less than the previous data point), then the previously stored peak data point in the increasing direction (i.e., the high peak data point) is paired with the previously determined peak data point in the decreasing direction (i.e., the low peak data point) to form a cycle pair. The structural fatigue life is then determined, in real time, from this cycle pair.
Instead, if the most recent data point in the current direction (e.g., increasing) either equals the most recent data point in that direction, or is greater than the most recent data point in that direction, then no change of direction is indicated. As such, no new cycle pair has yet been identified. The present invention does this until a change in direction is indicated. At that time a cycle pair is determined to exist, from which the structural fatigue life can be determined.
Essentially, the present invention continuously evaluates the current trend (increasing or decreasing) of the magnitude of the stress data. Once the trend reverses, a cycle pair comprising the most recent high and low peak data points is identified, stored in memory, and utilized in determining the fatigue life of the structure. The fatigue life is determined using various cumulative damage calculation methods. The cycle pair is commonly referred to as a xe2x80x9ctype III cyclexe2x80x9d. Once a cycle pair is determined, only the high and low data points comprising the pair need to be stored in memory.
The above and other objects and advantages of the present invention will become more readily apparent when the following description of a best mode embodiment of the present invention is read in conjunction with the accompanying drawings.